Is there a mental disease/disorder that has to do with someone constantly thinking about what others

The former could be a form of Social Phobia, and should be quite treatable.

The thing about feeling that someone else is in the house, I read somewhere, can be a problem with the brain's body regulation center, so that what you are actually sensing is yourself and your own presence, but your brain somehow is misreading it, leading to the misperception that someone else is there. Now, I don't want to worry you at all (and this is why I really advise going to see a psychiatrist or psychologist, and please remember that I am no professional and may totally not know what I am talking about) but that misperception condition can often be related to schizophrenia. Is there any history of that in your family? I don't know if it could be a beginning warning sign of onset? I also don't know how old you are, and how possible such an onset is likely to be.